An object detection method using background subtraction is known as a conventional object detection method (for example, refer to Japanese Unexamined Patent Application Publication No. 2010-256194). The object detection method using background subtraction detects a detection target object based on a difference between a background image (image as a reference for the absence of a detection target object) and a detection target image (image as a detection target to detect the detection target object). More specifically, an area where a difference value between the background image and the detection target image is above a predetermined threshold is determined as a detection target object. The object detection method using background subtraction is a simple method, and is generally used as an object detection method where stable background images can be obtained.
On the other hand, there is a method which performs Gaussian filtering on an image to smooth the image, and then performs Laplacian filtering on the smoothed image to obtain second derivatives of the smoothed image. This method is commonly called LoG filtering, and is often used, for example, to detect an edge of an image (detect an area where the intensity changes significantly).
This method using LoG filtering, while often used e.g. to detect an edge of an image, can be applied e.g. to the case of detecting a detection target object where the detection target object is assumed to be a hot body, and the detection target image is assumed to show a temperature distribution. More specifically, a detection target image showing a temperature distribution has negative second derivatives in an area where the temperature increasing rate decreases. Now consider the case where the detection target image showing a temperature distribution has a low pixel resolution, causing the detection target object to be in an area of several to ten and several pixels. In this case, the second derivatives of the detection target image are negative in the area where the detection target object is present. Thus, in this case, the detection target object can be detected by performing LoG filtering described above on the detection target image to obtain second derivatives of the detection target image, and by using the second derivatives. More specifically, an area where the second derivatives are lower than a predetermined threshold (0 or a predetermined negative value) can be determined as the detection target object.